488 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



spectra (Fig. 53) are extremely simple, consisting, in the case 

 of fourteen elements from aluminium to zinc, of two 

 principal lines, decreasing in wave-length and increasing in 

 frequency as the atomic weight increases. The increase of 



FIG. 53. HIGH-FREQUENCY SPECTRA. 



frequency is perfectly regular, the square-root of the frequency 

 increasing by equal amounts between consecutive elements ; 

 thus by selecting a suitable constant the square-roots of the 

 wave-lengths can be plotted out against a series of integral 

 numbers (Fig. 54). The exactness of the relationship is 



